Field of the Invention
The invention relates to a method for carrying out a rule-based OPC (optical proximity correction) with simultaneous scatter bar insertion.
In lithographic methods for fabricating structures of microelectronic components with the aid of projection exposure, ever finer structures must be generated on substrates.
The wavelengths utilized in projection exposure today are larger than the smallest dimensions of the structures to be generated on the substrate. Therefore, diffraction effects play a large role in the imaging of the structures. They effectuate a flattening of the intensity gradients at the edge of a structure by scattering light in the exposing of closely adjoining structures.
The emerging edge unsharpnesses and consequent dimensional distortions are principally dependent on the relative proximity of the structures on the substrate, the light wavelength, and the numerical aperture of the exposure device. Thus, two structures with the same dimensions will have different dimensions after exposure in one and the same exposure process (i.e. the same wavelength and numerical aperture), depending on how close these structures are to other parts of the exposed layout. This change of the dimensional relations in dependence on the relative proximity of the structures is called proximity effect.
A known technique for alleviating proximity effects is to carry out an optical proximity correction of the structure (OPC) before the mask is created. An OPC is carried out in order to guarantee that the finished structures on the substrate are actually the sizes provided in the layout. This cannot be assumed because size changes can occur during the fabrication of a microelectronic component (for instance due to the proximity effect). These are compensated by the OPC.
The OPC can be carried out by means of a simulation program or a rule-based software system. Utilizing simulation programs requires a substantial computing outlay, which is smaller given the application of rules for generating the correction of the structure. In rule-based OPC, a correction value is tabulated in dependence upon the dimension of parts of the structure (e.g. the width) and in dependence upon the spacing between two parts of a structure or spacing in between neighboring structures, and this value is utilized for correcting the layout data. The mask is created from this. Rule-based OPCs are preferred owing to the lower computing outlay.
U.S. Pat. No. 5,242,770 describes attaching thin lines between the structures in order to improve the sharpness of the imaging in lithography. These lines are so thin that they are not imaged on the substrate themselves. These lines, known as scatter bars, influence the intensity gradients at the edges of the structures, so that differences between tightly packed parts of the structure and more widely spaced parts of the structures are balanced out. The scatter bars are arranged parallel to parts of the structure at predeterminable intervals.
In general, semiconductor layouts are created with the aid of computer programs. In a prior art method, a series of data sets are formed in the course of the creation process. These data sets are referred to as layers and contain structures. The structures of one or more layers are written onto a mask, which are imaged onto a substrate by exposure (main structures). As explained above, the mask also contains scatter bars, which cannot be imaged onto the substrate by exposure owing to their small dimensions. But they support the imaging of the main structures.
In simulation-based OPC, scatter bars are taken into consideration as a proximity layer. The proximity layer basically represents structures which are taken into consideration in the correction of the main structure but which cannot be corrected themselves. For instance, the proximity layer can represent scatter bars between the main structures. When the main structure is subsequently subjected to simulation-based OPC, the positions of the scatter bars are also taken into account, but a correction (by simulation-based OPC) is only carried out at the main structure.
It would be desirable to subject the layer that is to be corrected to a rule-based OPC instead of a simulation-based OPC, because the computing outlay is very large for simulation-based OPCs owing to the increasing miniaturization of structures.
In rule-based OPC, the correction that is to be calculated depends on the spacings between parts of the main structure or the spacing between neighboring main structures. The problem is knowing whether a scatter bar is located in the interval between two main structures or in the interval between two parts of the main structure. But this is complicated by the fact that when scatter bars are generated, many need to be removed again, for instance because they cross other scatter bars. Given crossovers, larger structures could otherwise be generated on the mask from the thin lines of the scatter bars, which structures could be imaged onto the substrate by exposure. This is to be avoided.
If, then, the scatter bars were considered as proximity layers in a rule-based OPC, it would be necessary to distinguish between measured intervals between two parts of the main structure and those between a part of the main structure and a scatter bar, because in rule-based OPC the defining of a specific correction value depends on this. Without such discrimination, it would be impossible to provide unambiguous correction values.
The combination of a rule-based OPC method and scatter bars is thus not viable without further ado.